Many conventional RF communications systems exist. Users within these systems select a channel (if there is more than one channel available), monitor the channel for activity, and transmit when no one else is using the channel. Communication units within these systems include mobile and portable radios, capable of voice and/or data transmission. Some systems are very large, such as a public transportation operation. Such systems may include units in buses, trains, tow trucks, snowplows, security vehicles, and depots. In a large city, these vehicles and buildings are spread out over a large area. For example, each city may have ten divisions. Each division may have twenty to thirty depots. Each depot will have numerous tow trucks, snowplows, security vehicles, and buses or trains. These vehicles are dispersed among various bus routes or train routes near the depot.
When making a transmission on a large conventional system, usually all users will hear that transmission. Since only rare transmissions require all users to hear the transmission, many are bothered because they must listen to a message they do not need to hear, and further they cannot use the system for their own needs. Such a call is also an inefficient use of spectral resources. Some conventional systems employ spectrally-limited connect tones, but these tones are limited in availability and take additional time to reach the user. It is therefore reasonable to expect that such a large system will convert to a more spectrally-efficient trunked communications system, which provides fleet calls to reach a specific group of users. Converting to a trunked system is extremely costly, however, and funding is difficult to obtain in public transportation operations.
Generally, within a trunked system, the many users are organized into fleets, subfleets or groups, and individuals. Each user has a fleet, group, and an individual indentification (ID) code to provide a mechanism enabling group or fleet calls. Over-the-air protocols are used to program certain parameters of communication units, such as frequency assignments or the ID field, without bringing each unit out of the field for servicing, saving weeks or months of lost communication time. Trunked systems are unable to provide group data messages, however, and a data message for multiple units must be sent individually to each unit, which is a spectrally inefficient procedure. Many trunked systems also require permission to regroup units as well as acknowledgments from each communication unit involved, an inefficient procedure.
A large organization, such as a public transportation operation, with a conventional system needs the ability to address only a select group of communication units without disturbing users that are not involved in that call.
A method of communication that enables a conventional system to efficiently provide subfleet calls for both voice and data communications is desired.